A conventional light detecting apparatus operates such that a slit light is radiated toward a predetermined range. The light thus radiated is imaged by a CCD or the like, and the image pickup data is processed thereby to measure the distance to or the shape of an object existing in the predetermined range. One example of such an apparatus is disclosed in Japanese Patent Application Laid-Open No. 2000-97629. In that application a light radiation time or a light receiving time is finely adjusted in stages so that a received light amount of pixels of imaging means constitutes an optimum amount below a saturation amount.